Embryonic determination appears to be mediated by morphogenetic substances which are spatially localized in the egg and differentially partitioned into certain cell lineages during development. These substances are thought to influence the program of gene expression in the embryonic regions in which they are eventually deposited. There is evidence that an active component of some of the morphogenetic substances is maternal RNA. In the proposed research we plan to test the possibility of differential cytoplasmic distribution of maternal poly(A)plus RNA, a class of mRNA, in a model system, the ascidian egg. The ascidian egg exhibits cytoplasmic fields, already known to contain different concentrations of total poly(A)plus RNA, which delimit the presumptive fate of particular embryonic cell lineages. Our approach will be to produce probes of specific poly(A)plus RNA species by cDNA cloning in E. coli. The cloned ascidian DNA molecules will then be used as highly specific probes to test the localization of complementary poly-(A)plus RNA sequences in the cytoplasmic fields by in situ hybridization. Once the probes are prepared and characterized it is also planned to analyze the oogenetic origins of localized maternal RNA in both a temporal and spatial framework, explore how these localizations are maintained in the egg in relation to the cytoskeleton, analyze the persistence and eventual fate of these molecules during embryogenesis, and examine the identity and sub-cellular distribution of their translational products.